Fluoroelastomers having excellent heat resistance, oil resistance, and chemical resistance have been used widely for sealing materials, containers and hoses. Examples of fluoroelastomers include copolymers comprising units of vinylidene fluoride (VF2) and units of at least one other copolymerizable fluorine-containing monomer such as hexafluoropropylene (HFP), tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), vinyl fluoride (VF), and a fluorovinyl ether such as a perfluoro(alkyl vinyl ether) (PAVE). Specific examples of PAVE include perfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether) and perfluoro(propyl vinyl ether). Other fluoroelastomers include copolymers comprising units of TFE and units of perfluoro(methyl vinyl ether).
In order to fully develop physical properties such as tensile strength, elongation, and compression set, elastomers must be cured, i.e. vulcanized or crosslinked. In the case of fluoroelastomers, this is generally accomplished by mixing uncured polymer (i.e. fluoroelastomer gum) with a polyfunctional curing agent and heating the resultant mixture, thereby promoting chemical reaction of the curing agent with active sites along the polymer backbone or side chains. Interchain linkages produced as a result of these chemical reactions cause formation of a crosslinked polymer composition having a three-dimensional network structure. Polyhydroxy compounds are commonly employed as curing agents for fluoroelastomers. Such nucleophilic curatives require an acid acceptor (e.g. a divalent metal oxide and/or divalent metal hydroxide) to be activated.
However, cured fluoroelastomer articles containing acid acceptors may exhibit unacceptably high volume swell, that can lead to seal failure, when articles are exposed to carboxylic acids such as acetic acid, despite having excellent resistance to much stronger mineral acids such as sulfuric acid. Exposure to carboxylic acids may occur in various end uses including chemical industry applications and automotive applications such as exhaust gas recirculation (EGR) systems and biofuel management systems.
Volume swell and surface damage of polyhydroxy cured fluoroelastomer articles due to exposure to carboxylic acids may be lessened by reducing the amount, or eliminating, metal oxides and metal hydroxides from the articles. However, the curing rate of such articles is significantly reduced. Thus, there is a need to provide polyhydroxy curable fluoroelastomer compositions that cure rapidly and which still are resistant to volume swell when exposed to carboxylic acids.